A widely used technique for force detection and measurement employs a mechanical capacitive readout force transducer having a capacitive output proportional to the force applied. In one such mechanical transducer, one or more capacitors are formed between an instrument frame and a proof mass suspended by a flexure. A force applied to the proof mass along a particular axis will cause displacement of the proof mass relative to the frame, which varies the capacitive output of the capacitors. The force applied to the proof mass is quantified by measuring a resultant change in capacitance.
Such a micro-accelerometer is a combination of mechanical structure comprising the proof mass, suspension flexure, and fixed instrument framework, and electrical structure comprising capacitor electrodes, current-feed connections, and external circuitry forming a capacitor circuit.
More precisely, a micromachined proof mass is connected by a flexure to a fixed instrument frame forming a part of the framework of the transducer. A capacitor is formed between one or more electrode surfaces of the hinged proof mass and opposing surfaces of cooperating fixed electrodes of the instrument frame. Movement of the proof mass electrode surfaces relative to the cooperating fixed electrodes changes the value of the capacitor of the capacitor circuit. This variation in the capacitor value depends on the relative movement of the proof mass by forces, i.e., acceleration, applied to the proof mass. A change in the measurement of the capacitor therefore represents an acceleration measurement.
According to one well-known technique, the proof mass's electrodes and cooperating fixed electrodes are formed having intermeshing comb-like fingers wherein a large quantity of substantially parallel-plate capacitors are formed between the opposing surfaces of the fixed electrode and proof mass fingers.
Capacitive readout force transducers employing such intermeshing comb-like fingers have been fabricated from a body of semiconductor material, such as silicon, as microelectromechanical systems, or “MEMS,” integrated micro devices or systems combining electrical and mechanical components fabricated using integrated circuit (IC) batch processing techniques.
In the most general form, MEMS consist of mechanical microstructures, microsensors, microactuators and electronics integrated in the same environment, i.e., on a silicon chip. MEMS is an enabling technology in the field of solid-state transducers, i.e., sensors and actuators. The microfabrication technology enables fabrication of large arrays of devices, which individually perform simple tasks but in combination can accomplish complicated functions. Current applications include accelerometers, pressure, chemical and flow sensors, micro-optics, optical scanners, and fluid pumps. For example, one micromachining technique involves masking a body of silicon in a desired pattern, and then deep etching the silicon to remove unmasked portions thereof. The resulting three-dimensional silicon structure functions as a miniature mechanical force sensing device, such as an accelerometer that includes a proof mass suspended by a flexure. Existing techniques for manufacturing these miniature devices are described in U.S. Pat. No. 5,006,487, “Method of Making an Electrostatic Silicon Accelerometer” and U.S. Pat. No. 4,945,765 “SILICON MICROMACHINED ACCELEROMETER,” the complete disclosures of which are incorporated herein by reference.
High aspect ratio MEMS or “HIMEMS” is one known process for producing such MEMS devices, including MEMS accelerometer devices. HIMEMS permits fabrication of intricate device designs in two dimensions, but requires a fixed device thickness, on the order of a cookie cutter. Capacitive readout in-plane accelerometers fabricated as HIMEMS devices may includes a proof mass suspended on flexures with a quantity of fingers formed along its edges, the fingers on the proof mass intermeshing with cooperating electrode fingers on a fixed structure to form a quantity of capacitors therebetween. As the proof mass moves in response to an applied force, the intermeshing fingers of the proof mass approach or recede from the fingers of the fixed structure, whereby capacitance therebetween increases or decreases. The change in capacitance between fingers is a measure of the force applied to the proof mass, which can be interpreted as an acceleration signal. One problem with such capacitive readout in-plane accelerometers is that, when fabricated as HIMEMS devices, forces applied out-of-plane relative to the intermeshing fingers causes out-of-plane separation or “de-meshing” of the fingers, which decreases the inter-finger capacitance and registers as a decrease in the applied force or acceleration. This out-of-plane separation or “de-meshing” of the fingers is a device “cross-axis sensitivity” that results in a decrease of inter-finger capacitance whether the out-of-plane force or acceleration is applied positively or negatively.